The diurnal variation in the cosmic-ray intensity recorded by a network of Japanese multidirectional muon telescopes during 18 years from 1978 to 1995 was analyzed to examine the long-term variations in the local heliospheric modulation parameters. It was found that the upper-limiting rigidity of the variation changes between 100GV to 300GV with two maxima in 1982 and 1989. It is also clear that the radial density gradient varies with a good correlation with solar activity, while the parallel mean-free-path of the cosmic-ray diffusion varies with an anti-correlation with solar activity, both in accord with the conventional modulation theory incorporating the convection and diffusion processes. The bi-directional latitude gradient, on the other hand, shows a clear 22-year solar magnetic cycle as predicted by the drift theory for cosmic-ray transport. The mean value of lambda_<||> was larger in the period of the negative solar magnetic polarity than in the positive polarity period but the difference was not significant. There was no polarity dependence found in G_<gamma>. We also examined the variation of the unidirectional latitude gradient by analyzing the "Toward-Away" solar diurnal variations observed by 37 directional telescopes in the network. In our analysis, we took into account not only the NS symmetric anisotropy (A_<sym>) but also the NS anti-symmetric anisotropy (A_<anti-sym>), which was first observed by the Nagoya surface muon telescope in 1971-1979. The phase of the yearly mean A_<sym> was found at -0300 or -1500 hours local solar time depending on the year, while the phase of A_<anti-sym> was always found at -1500 hours in the northern hemisphere. G_<rheta> derived from A_<sym> showed no clear variation related to the 11-year solar activity-or 22-year solar magnetic-cycles, but it remained positive after the late 80's implying a higher population of cosmic-rays in the southern hemisphere below the heliographic equator.